1. Field of the Invention
This invention relates to an improved method for purification of a cumene recycle stream and more particularly this invention relates to methods for removing cumene hydroperoxide and 2-phenyl-2-propanol impurities from a cumene recycle stream in order to reduce the loss in selectivity to cumene hydroperoxide resulting from peroxidation of recycle cumene containing impurities. The impurities can be removed by treatment with a clay catalyst in one embodiment and, in another embodiment are removed by a catalyst comprising phosphoric acid on silica.
2. Description of Related Art
The cumene process is used to manufacture more than 90% of the phenol produced today. In the process, propylene is reacted with benzene to produce cumene. ##STR1## After purification, the cumene is reacted with either air or oxygen to produce cumene hydroperoxide. ##STR2##
Careful adjustment of reaction conditions is necessary to obtain good reaction rates without loss of yield. In most industrial processes, the conversion of cumene to products is held to 20-40%. Under the best conditions yields of over 90% to cumene hydroperoxide may be obtained.
Most of the unconverted cumene is removed from the oxidation product by vacuum distillation and recycled to the oxidation reactor. The concentration of cumene hydroperoxide is then 75-85%.
The next stage in the process is the cleavage of the peroxide to phenol and acetone which can be represented as follows: ##STR3##
The product from the cleavage stage contains phenol, acetone, cumene, acetophenone, 2-phenyl-2-propanol and other minor byproducts. Phenol and acetone are separated by distillation.
The cumene present in the cleavage step is distilled but still contains impurities. It is combined with the cumene isolated from the concentration step. This combined product is recycled to the oxidation reactor, but contains various impurities such as cumene hydroperoxide, 2-phenyl-2-propanol, acetophenone, .alpha.-methyl styrene and phenol. .alpha.-Methyl styrene and phenol are especially detrimental in the oxidation. These two impurities are difficult to remove by distillation since 2-phenyl-2-propanol decomposes to give .alpha.-methyl styrene and cumene hydroperoxide may decompose to give phenol and acetone during distillation.
There is an overview of the technology and economics relating to the production of phenol in Chem. Systems Report No. 79-2, September, 1979, p. 155. It is noted that an important factor influencing the oxidation rate is the purity of the cumene feed, since small amounts of inhibitors, such as sulfur compounds, phenols, aniline or styrenes present in the feed may break the reaction chain by tying up free radicals. Other compounds that catalyze the decomposition of the hydroperoxide are soluble compounds of copper, cobalt and manganese. Acceptable oxidation and conversion rates can be achieved with normal industrial cumene without extensive feed pretreatment, but this generally leads to lower conversion efficiencies and increased side reactions. In the market recently the requirements for improved conversion efficiencies have resulted in the tightening of cumene feed specifications. Purities as high as 99.9 weight percent are now commonly required for the production of chlorination-grade phenol. An alkali wash cumene feed pretreatment is also generally beneficial.
In Kirk-Othmer Encyclopedia of Chemical Technology, 7, 373(1982) there is a discussion of various methods for producing phenol and of the economic aspects related thereto. At page 380, second paragraph it is noted that the USP specification for phenol requires a phenol content of not less than 98%.
The following early patents provide background for the oxidation of aromatic hydrocarbons:
U.S. Pat. No. 2,447,400 PA1 U.S. Pat. No. 2,547,400 PA1 U.S. Pat. No. 2,632,772 PA1 U.S. Pat. No. 2,632,773 PA1 1. An acidic clay catalyst PA1 2. A phosphoric acid on silica catalyst
to produce products such as acetophenone, peroxides of alkylbenzenes, etc.
U.S. Pat. No. 2,706,708 discloses a process for separating cumene from cumene hydroperoxide, wherein the hydroperoxide is made substantially free of dialkylarylmethane by stripping the dialkylarylmethane from dialkylaryl methyl hydroperoxide and passing it through a separation zone.
An improvement in a method for producing cumene hydroperoxide is disclosed in U.S. Pat. No. 3,049,477. The improvement comprises fractional distillation of the oxidation reaction mixture in a still to obtain an overhead fraction comprising cumene, water and organic acids and a bottoms product comprising cumene hydroperoxide which occurs in a concentration higher than it occurred in the oxidation mixture, the improvement comprising recycling the total overhead fraction and recycling at least the cumene to the oxidation step and adding to the still as a liquid reflux a fresh cumene characterized by being substantially dry and having a pH not less than 5.
U.S. Pat. No. 3,092,587 demonstrates the use of thin film and reflux measures to produce high percentage solutions of organic peroxides from low percentage solutions of alkylated aromatic hydrocarbon organic peroxides.
It was disclosed in U.S. Pat. No. 3,141,046 that an effective catalyst for the oxidation of cumene to hydroperoxide is an alkali salt of a carboxylic acid of the general formula: ##STR4## where R.sub.1 represents an alkyl or aryl group and R.sub.2 represents a straight or branched-chain trivalent saturated aliphatic hydrocarbon group. The invention disclosed in this patent comprises oxidizing cumene by treatment at an elevated temperature with molecular oxygen in the presence of the above-described alkali metal salt of a carboxylic acid.
U.S. No. 3,290,384 discloses a method for the oxidation of aralkyl hydrocarbons such as isopropyl benzene to aralkyl hydroperoxides in the presence of a specific class of heavy metal catalysts.
In U.S. No. 3,519,690 there is disclosed a process for the separation of aliphatic or aromatic organic hydroperoxides from their liquid mixtures by thermal diffusion.
In the invention of U.S. No. 3,647,886 certain hydrocarbyl hydroperoxides are formed by contacting a hydrocarbyl hydrocarbon with an oxygen-containing gas in the presence of KF.
There is disclosed in U.S. No. 3,833,663 a process for the oxidation of cumene to cumene hydroperoxide in which cumene is contacted in the liquid phase in the presence of cumene hydroperoxide at a temperature in the range of 40.degree. to 140.degree. C. with a gas containing free oxygen over a catalyst comprising a fluoride of an alkali or alkaline earth metal.
In Liquid Phase Oxidation of Hydrocarbons, Emanual, N. M., et al., Plenum Press (1967), Chapter IX, titled "The Oxidation of Individual Hydrocarbons", it is noted at page 314 that benzoic acid accelerates the decomposition of the hydroperoxide into radicals. Apparently the addition of certain acids inhibits the oxidation of cumene, causing decomposition of the peroxide into phenol and acetone. As little as 0.02% by weight of formic acid almost completely inhibits the oxidation of cumene, Ibid., p. 315.
It was found the pH of the medium had an additional influence on the kinetics of the oxidation of cumene. For example, alkali-containing compounds were found to have an accelerating action. In addition, the inhibiting action of phenol on the oxidation is greatly weakened in the presence of alkali, Ibid.
Emanual, et al. provided evidence to indicate that in the presence of an alkali, decomposition takes place by the overall equation: EQU RH+ROOH.fwdarw.2ROH
It was concluded that the accelerating effect of alkali is connected, not only with the neutralization of acidic products, but also with an increase in the rate of initiation of the chains.
In a strongly alkaline medium (5-10% NaOH) the oxidation of cumene does not go selectively, although the rate is high and the yield of hydroperoxide does not exceed 50% of the oxygen absorbed. Acids formed in the oxidation of cumene at 85.degree. C. in an aqueous emulsion lower the pH of the emulsion and retard the reaction. At a pH of about 4-7(neutral or acidic) oxidation takes place slowly, similar to homogeneous oxidation at this temperature.
It would be a substantial advance in the art in this field if an efficient method were available which would provide a way to remove 2-phenyl-2-propanol and cumene hydroperoxide from the cumene recycle stream so that subsequent distillation would allow production of the recycle cumene essentially free of detrimental impurities.